We've been taught that when spring begins, the day will have equal daylight and dark hours. In other words, that there should be 12 hours of daylight and 12 hours of dark on the first day of spring.
Spring begins on Monday, Wednesday, March 20 at 4:02AM.
Tucson's day of equal daylight and dark hours, though, is on Friday, March 16, 2012.
Why is this? Why isn't the day of "equality", as we'll refer to the day with equal daylight and dark hours, on the first day of spring?
First, it's important to note that Tucson is not the only location not to have their day of "equality" on the first day of spring. The following is a list of sample cities in the northern and southern hemispheres and their daylight hours on the first day of spring, 2013. In "theory", there should be 12 hours of daylight on the first day of northern hemisphere spring, regardless of location.
Barrow, Alaska: 12hr10min.
Vancouver, B.C., Canada: 12hr8min.
Tucson, AZ: 12hr6min.
Mexico City, Mexico: 12hr, 6min.
Manaus, Brazil: 12hr5min.
Rio de Janeiro, Brazil: 12hr9min.
Figures 1: Some sample cities and their daylight hours on the first day of spring, 2012.
One reason for this displacement of the day of "equality" away from the first day of spring is because the sun's light is actually bent through our atmosphere. This results in us seeing the sun rising BEFORE it actually hits the horizon and setting AFTER it has already hit the horizon.
Figure 2: The sun's light is bent around the horizon due to our atmosphere, creating the illusion of sunrise being earlier and sunset being later than they actually are.
Another reason is that sunRISE is when the top of the disc of the sun peeks over the horizon and sunSET is when the top of the sun's disc drops below the horizon. This creates a bit of a discrepancy because that means the time it takes for the full disc of the sun to set is counted in the daylight hours, adding to what would otherwise be a possible day of equal daylight and night.